As circuit size decreases in many mobile communications devices, and associated plastics housings and the like reduce in size, mobile radio handsets are also decreasing in size. One item of a radio communications device which cannot easily be reduced in size, however, is the antenna. Typically the antenna is one half or one quarter of a wavelength in length along at least one axis and as such cannot easily be reduced.
An antenna radiates electromagnetic waves when there is an acceleration of charge through the conductor. This produces a magnetic field, which then produces electromagnetic (EM) radiation. One type of antenna known to those skilled in the art is the resonant dipole antenna 100, depicted at FIG. 1. A radio frequency (RF) source 120 is depicted at the center of the conductor 110 for providing an RF signal resonating at a given frequency (e.g., 5 GHz). The conductor extends out from either end of RF source 120 by ¼ wavelength (¼ λ).
The magnitude of the instantaneous current flowing through the conductor is depicted by the curved line to the right of the antenna. As depicted, the current flow is at a maximum at the center of the conductor 110 and gradually reduces as the ends of the conductor 110 are approached. The circles depict the direction of the magnetic field produced by a current flowing in the upward direction. The magnetic fields for the upper and lower halves of the conductor 110 are depicted as being in the same direction. This signifies that the EM radiation from each half is in phase.
Turning to FIG. 2, the dipole antenna of FIG. 1 is depicted as being bent in half to reduce its vertical profile. The FIG. 2 antenna 200 is known in the art as a double inverted-L antenna. Here, the antenna 200 resonates at the same frequency (e.g., 5 GHz) as the FIG. 1 dipole antenna 100, and the current magnitude remains unchanged from that of the FIG. 1 antenna 100. The main difference between the FIG. 2 antenna 200 and the FIG. 1 antenna 100 is that the magnetic fields produced by the two horizontal portions 220, 230 are now 180-degrees out-of-phase and cancel each other out. As a result, there is virtually no EM radiation from the horizontal portions 220, 230 of the antenna 200; only the vertical portion 210 radiates, thereby greatly reducing the radiation resistance of the antenna 200 from that of the FIG. 1 dipole antenna 100. A reduced radiation resistance translates to the need for a higher antenna current to radiate the same RF energy. Accordingly, there is a need in the field of radio communications for a low profile antenna designed to provide a vertically short profile while exhibiting a relatively high radiation resistance, wide bandwidth, and gain over a simple short conductor.